This uses the source spans exposed by the yaml package to associate pubspec
errors visually with the section of the pubspec that caused them.

R=rnystrom@google.com
BUG=14999

Review URL: https://codereview.chromium.org//351703004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@37675 260f80e4-7a28-3924-810f-c04153c831b5